Use an email which MX records returns FALSE but can provide "A" record. Then isValid() always return FALSE.

$validator->isValid('USER@EMAIL_ONLY_WITH_A_RECORDS');

TIP: On file "Zend/ValidateEmailAddress.php" function line 442: Function "getmxrr" always return false because there is no "MX" records, and then never executes the code that checks the "A", "AAAA", "A6" records.